# Noise-induced Coherent Dynamics of Multilevel Quantum Systems Driven by Incoherent Light: A Case Study of the Three-level V-system

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## Authors

Koyu, Suyesh

## Issue Date

2020

## Type

Dissertation

## Language

## Keywords

detuning , Fano coherence , Lorentzian , non-equilibrium steady-state , quantum coherence , resonance fluorescence

## Alternative Title

## Abstract

The time evolution of quantum coherence in noisy quantum systems interacting with a thermal environment is of interest to many areas of quantum physics and technology, including quantum information processing and quantum sensing. A three-level V-system which comprises two quasi-degenerate excited states and a common ground state serves as a minimal model of multilevel quantum systems, such as photosynthetic light-harvesting complexes and photovoltaic devices. We study the dynamics of noise-induced Fano coherences between the excited states of the V-system driven by incoherent radiation and classify the dynamical regimes into underdamped, overdamped, and critical. In the underdamped regime ($\bar{n} < \Delta/\gamma$), where the ratio of the excited-state splitting $\Delta$ to the spontaneous decay rate $\gamma$ is greater than the average photon number $\bar{n}$ in the thermal radiation field, the coherences oscillate and decay exponentially with a lifetime $1/(\bar{n} \gamma)$. In the overdamped regime ($\bar{n} > \Delta/\gamma$), the noise-induced coherences exhibit a long-lived quasi-steady state with the lifetime $\tau_{c} = 1.34 \frac{\bar{n}}{\gamma} \big(\frac{\Delta}{\gamma}\big)^{-2}$. \par Furthermore, we explore the quantum dynamics of the V-system illuminated by polarized incoherent radiation. We find that polarized incoherent driving of the V-system leads to the formation of non-equilibrium coherent steady-states. We propose a scheme for the experimental detection of steady-state Fano coherences in Ca atoms in magnetic fields driven by polarized incoherent light. We find the signatures of steady-state Fano coherences in the deviation of excited-state populations from their values in thermodynamic equilibrium. \par In addition, we consider the dynamics of one-photon Fano coherences between the ground and excited states of the V-system. We find that these coherences cannot be generated by incoherent driving, which is in contrast to the case of two-photon Fano coherences. Nevertheless, we observe an interesting coherence transfer phenomenon between the two one-photon transitions in the V-system initially excited by a coherent laser pulse. This coherence transfer is caused by the coupling between the one-photon transitions induced by Fano interference. Besides, we study the resonance fluorescence spectrum of the radiation emitted by the V-system in the far-field region in both the weak and strong pumping limits. The emission spectra are found to be a sum of two Lorentzians and an interference term. Significantly, in the underdamped regime, we find the emission line splits into two distinct peaks with a sharp dip centered at zero detunings. We find that Fano coherences lead to emission-line narrowing, which is the most pronounced in the underdamped regime, where the excited-state splitting is less than the radiative decay rate.